1. Field of the Invention
The invention relates generally to the testing of magnetic media and drives, and specifically to servowriting a previously untested floppy disk first, then certifying the disk such that the locations of any and all media defects can be identified and logged.
2. Description of the Prior Art
There are two principal operations involved in the prior art manufacture of magnetic media and disk drives. First, a certification of the quality of the media is done with specialized equipment. Second, a servowriting operation is done that places reference information on the surface of the media. ("Servowriting" places data on a disk that is used to for precise positioning of the heads on the media.) Certification usually means that 100% of the surface of the media is tested. In servowriting, the reference information is placed in very specific locations. The prior art practice has been to test and certify the media first and then write the servo information. Some prior art systems put down a magnetic index mark and record displacement angles of media defects with respect to the magnetic index mark. Then a physical index, typically a small punched hole, is placed such that identified media defects will not coincide with the physical index mark. The present invention puts the reference information down first, and then does the certification step. To do this means that there must be much more intricate control of the test process than had previously existed to avoid overwriting the reference information. But the advantage is media defects are very rapidly and precisely located.
As the capacity and performance of hard disks continue to advance, the number of tracks per inch (TPI) grows higher and higher. Higher density requires the use of servo positioning systems to accurately position the heads. Servowriters are required to properly write the servo position information on the disk.
Servowriting is a critical production step for a number of reasons. Since the drive can not be functionally tested until the servo information is in place, many production problems are discovered during servowriting or immediately after. If a problem does arise that prevents proper servowriting, the cause of the problem must be quickly determined. Diagnosis of the problem is often very difficult. Uncertainty about servowriter performance or reliability compounds the problem. Problems that affect more than a small percentage of hard disk assemblies (HDAs) may shut down production. Servowriters have a significant impact on development schedules too. The design of the servowriter is usually on the critical path to product introduction. Unless a servowriter is very similar to one that is already developed and successfully in use, the design and development process can be very arduous, delay production of the drive, and even delay the development process of the drive itself.
Magnetic disks and especially floppy disks consist of ferrite compounds on a substrate. Common floppy disks comprise MYLAR coated on both sides with ferric oxide. It is a common practice in the disk media industry to initially manufacture all floppy disks in a single batch, and then grade them into various classifications according to how well they test. The disks that are able to accept error free recordings in the highest density formats are certified for premium use, and carry the highest prices. Some disks will accept less dense formats, and so are certified as a standard, or economy grade. And, of course, those disks that cannot accept even the simplest of formats are discarded as waste.
Magnetic disk drives and their media have advanced to the point that even small media defects can cause serious data errors, or other problems, because the storage densities are so high. Track-to-track densities have now exceeded 2000 tracks-per-inch (TPI). Some media defects can be large enough to prevent using a particular disk in the industry standard high density ("HD") or in very high density ("VHD") format, and yet the same defects can still be minor enough that the same disk can be used in the (older) single density ("SD") or double density ("DD") formats . Very often defects occur only on one side of the media, so such a disk could also be certified and sold as single sided "SS," as opposed to double sided ("DS"). Disk media certifiers have evolved to test and grade floppy disks.
Read/write disk recording heads have been used in the prior art for both servowriting and certification functions. Single gap heads, using five to 60 milliamp currents and a center tap are typical. Floppy disk drives typically employ heads that are meant to contact the media during normal use. Various hard disks, such as Winchesters, literally fly the heads on an air cushion very close to the spinning media. The "flying height" reference (head to disk) varies from one disk type to the next. Pre-recorded servotracks need to have standardized servo track information recorded in them. The bit patterns within such servotracks can be single pulse or dipole. The issues related to servotrack recording include index-to-servo pattern tolerance, track-to-track timing tolerance, absolute track zero tolerance, and the so-called "D50" measurement. "D50" is a measure of high density recording performance. Usually, servo pulses are at lower density, so D50 is not directly related. Test programs should take into account such things as media errors, head checks, calibration checks, etc.
Various methods have developed in the prior art for measuring sector average amplitude (SAA) and the setting of thresholds for missing pulse, over-amplitude pulse, high frequency modulation, and bit shift. A prior art method of charging a capacitor with a given time constant for SAA has not been accurate enough, and some have suggested sampling the envelope to obtain a more accurate average. Methods of using automatic gain control (AGC) have also been used.
A significant problem with prior art disk media certification is that the locations of defects are not known except in the crudest of ways (e.g., the side and distance from an index mark).